on August 15, 2026
The Biophilia Hypothesis and Evolutionary Rationality
Shinrin-yoku, a term coined in 1982 by Tomohide Akiyama (then director of the Japanese Forestry Agency), translates as "immersing oneself in the atmosphere of the forest, experiencing it through all the senses". In a country like Japan, where two-thirds of the territory is covered by forests, this practice has been structured as a branch of preventive medicine within forest therapy.
The biological basis explaining the effectiveness of this practice finds its foundation in the Biophilia Hypothesis, formulated by biologist Edward O. Wilson in 1984. Since the genomic and physiological organization of the human species has been shaped and coordinated over millennia within natural ecosystems, the human body retains a genetic predisposition toward plant environments. The tuning between human sensory receptors and the environmental frequencies of nature promotes homeostasis, reducing the alarm responses of the hypothalamus-pituitary-adrenal axis typical of urbanized environments.
Neuroendocrine and Cardiovascular System Modulation
Clinical studies led by Chiba University in Tokyo and other Japanese research centers have allowed for the evaluation of the physiological effects of forest therapy through neurobiological and vascular markers:
1. Reduction of Stress Hormones: Active and contemplative sessions in the forest induce a suppression of sympathetic nervous system activity and a concomitant activation of the parasympathetic system. This translates into a marked reduction in serum and salivary levels of cortisol, adrenaline, and noradrenaline.
2. Hypotensive Effect: The measurement of hemodynamic parameters in hypertensive subjects highlights a significant reduction in systolic and diastolic blood pressure, as well as resting heart rate, demonstrating a protective action on cardiovascular load.
Immunomodulation Mediated by Phytoncides and Terpenes
One of the most relevant discoveries concerns the immune system response induced by the inhalation of phytoncides. Volatile organic substances produced by plants for defense against pathogens and parasites, phytoncides are rich in aromatic compounds such as terpenes (α-pinene, β-pinene, D-limonene, camphene).
The inhalation absorption of these molecules produces a direct impact on the body's defenses:
· Activation of Natural Killer (NK) Cells: Increase in the absolute number and cytotoxicity of human NK cells.
· Expression of Anti-Cancer Proteins: Up-regulation of intracellular levels of cytolytic proteins such as perforin, granzyme A, and granzyme B, essential in immune surveillance and anti-neoplastic response.
Unlike walks in urban settings, which do not bring changes in immune status, the immunostimulatory effect of forest exposure persists for several days after the end of the forest stay.
Soil Microbiota and Negative Ions: The Role of Mycobacterium vaccae
The effectiveness of Shinrin-yoku also includes the interaction with the earth matrix and the electrostatics of the air:
· Mycobacterium vaccae: This saprophytic bacterium widespread in soil and vegetation, coming into contact with humans through inhalation or skin contact, acts on the immune response by stimulating the production of anti-inflammatory cytokines. This process activates populations of serotonergic neurons in the central nervous system, inducing an improvement in mood and reducing anxiety states.
· Negative Aeroions (Environmental Anions): Near waterfalls, streams, and dense forest canopies, the air presents a marked concentration of negative ions. The inhalation of negative aeroions stimulates mucociliary clearance, optimizes pulmonary gas exchange, and promotes redox balance, reducing oxidative stress and supporting cognitive clarity.
Bibliographic References and Clinical Studies
1. Wilson, E. O. (1984). Biophilia. Harvard University Press.
2. Li, Q. et al. (2007). In defence of trees: Phytoncides (wood essential oils) induce human natural killer cell activity. International Journal of Immunopathology and Pharmacology, 20(2), 3-8.
3. Park, B. J. et al. (2010). The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan. Environmental Health and Preventive Medicine, 15(1), 18-26.
4. Li, Q. (2010). Effect of forest bathing trips on human immune function. Environmental Health and Preventive Medicine, 15(1), 9-17.
5. Lowry, C. A. et al. (2007). Identification of an immune-responsive mesolimbocortical serotonergic system: Application to antidepressant-like effects of Mycobacterium vaccae. Neuroscience, 146(2), 756-772.
6. Miyazaki, Y. et al. (2011). Preventive medical effects of nature therapy. Nihon Eiseigaku Zasshi (Japanese Journal of Hygiene), 66(4), 651-656.
